1. Field of the Invention
The present invention relates to a radiation system, a lithographic apparatus comprising such a radiation system and a method of aligning a discharge axis of a discharge radiation source with respect to optics of a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A radiation source for EUV radiation may be a discharge plasma radiation source, in which a plasma is generated in a substance (for instance, a gas or vapor) between an anode and a cathode and in which a high temperature discharge plasma may be created by Ohmic heating by a (pulsed) current flowing through the plasma. Further, compression of a plasma due to a magnetic field generated by a current flowing through the plasma may be used to create a high temperature, high density plasma on a discharge axis (pinch effect). Stored electrical energy is directly transferred to the plasma temperature and hence to short-wavelength radiation. A pinch may allow for a plasma having considerably higher temperature and density on the discharge axis, offering an extremely large conversion efficiency of stored electrical energy into thermal plasma energy and thus into EUV radiation.
In an EUV discharge source the pinch may be triggered by a laser beam on the electrodes. However, even when the laser beam is constantly pointed at a fixed position, the exact origin of the pinch may vary. As a consequence the resulting pinch and its axis may vary. This may result in inaccurate aligning of the radiation beam with respect to the optics of a lithographic apparatus receiving the beam.